Mesh in front of Grease Pencil: how to remove the outline?
It is a known issue: https://developer.blender.org/T78795
The workaround mentioned on the bug report is to disable anti-aliasing for EEVEE (Film &gt; Filter Size = 0) and for Grease Pencil (Anti-Aliasing Threshold = 0). Which in turn means you'd have to render double the size then downscale the result to get anti-aliasing back.
EDIT: I came across a way to do this without losing GPencil anti-aliasing. Basically it requires isolating GP and Meshes in different View Layers and compositing them back together with a Z Combine node (as explained in my answer here), and disabling the node's &quot;Z anti-aliasing&quot;. You still have to set EEVEE's Filter Size to zero but at least the edges of GPencil objects stay smooth without doubling render size.